The invention relates to a method for the production of a heat storage means, more particularly in the form of a latent heat storage means, for vehicle heating systems run on heat from the engine, comprising a housing, which is made up of an outer container and in inner container arranged in spaced relationship to the outer container so as to include an insulating zone between them, a heat storage core arranged in the inner container and having at least one chamber for a storage medium in it, said chamber being separated by a partition wall from at least one flow path for a heat transfer medium, and an inlet duct and outlet duct for the heat transfer medium, such ducts being connected with the flow path and extending outwards through the insulating zone, the insulating zone having been baked out after the production of the housing for degassing and having been evacuated, and to a heat storage means for performing the method.
In the case of vehicle heating systems there is the general aim of being able to store heat overnight so that when starting up in the morning there is sufficient stored heat until the engine cooling water has attained its operating temperature. Taking into account the normal requirements in vehicle construction as regards low weight and a low overall size, the quality of the insulation desired in the heat storage means may only be attained by having a double-walled housing, which encloses the storage core on all sides and has an insulating vacuum in its double walling. In this respect it may be a question of a high vacuum or of insulation in the form of microporous materials as for instance powders or fibers, the space between them being additionally evacuated. Therefore housings comprising an outer container and an inner container are utilized, an insulating zone being present between, through which the inlet duct and the outlet duct extend for the heat transfer medium. Therefore thermal losses may be caused by convection and thermal conduction in the heat transfer medium, for instance in the form of the engine cooling water or of the engine exhaust gas. A heat storage means with such insulation is described in the German patent publication 3,614,318 A for instance.
The effectiveness of the insulation of such heat storage means is dependent on the one hand on the use of vacuum technology, which has long been known, and on the other hand on the sealed design of the insulating vessel with its inner and outer containers so that the thermal conduction by solid structures is limited to the inlet and outlet ducts and to the means supporting the inner container in the outer container. The conduction of heat via the inlet and outlet ducts with a small cross section and long paths may be kept relatively low.
After the mechanical part of the production of such a heat storage housing the desired insulating effect is able to be achieved by evacuation of air from the insulating zone within a few minutes. This insulating effect is however not permanent, because it is possible for materials to be absorbed on the surfaces, which delimit the insulating zone, of the outer and inner containers, of any radiation shields positioned in the insulating zone, on microscopic insulating materials and also on the essential fittings in the insulating zone, that is to say the inlet and outlet ducts and on the means supporting the inner container, such materials evaporating in the course of time so that the pressure in the insulating zone is increased and the insulating effect of the vacuum is reduced.
For this reason vacuum-insulated vessels are degassed for a prolonged period of time after the mechanical production of the equipment, the insulating zone being continually evacuated. In order to reduce the degassing time to for instance 24 hours the insulating vessel is heated to an elevated temperature during evacuation, this being termed baking out. Experience has shown that an increase in the temperature of 10.degree. C. results in halving the degassing time.
It is furthermore known that a significant source of contamination in vacuum vessels is water adsorbed on the walls. For the removal of the water three distinct temperature stages are possible that is to say approximately 120.degree. C. with low rates of evaporation, 180.degree. C. with very evaporation rates and approximately 360.degree. C. with a practically 100% evaporation.
It is also known that during the degassing the long term effect is dependent on the minimum temperature which is reached on the surfaces in the insulating zone. It follows from this that for a given long term effect all surfaces defining the insulating zone will have to reach the minimum temperature or to exceed it.
For the full scale application to automobile construction a long term effect of the vacuum is necessary, which requires a baking out in the second stage range at 180.degree. C. The operational temperature of heat storage means of this type is however at 90.degree. C. and the maximum temperature necessary is 125.degree. C. The bake out temperature necessary for the production of a good vacuum is consequently substantially above the operational temperature which is later to be expected. The result of this is that there is the problem of thermal damage to the heat storage means during the process of production, as for instance owing to increased vapor pressure of the heat storage media employed, or owing to degradation of the supporting parts in the heat storage core, for instance synthetic resins, due to thermal expansion.